1. Field of the Invention
The invention relates to a file management method and a hierarchy management file system for optimally allocating files corresponding to characteristics of a file system.
2. Description of Related Art
Currently, storage devices having various performances are being developed and volumes composing the storage device have differences in their performances. In general, a high-performance volume is expensive and is unable to hold a large capacity. Meanwhile, a low-performance volume is inexpensive and can hold a large capacity.
Still more, there is a tendency that although data stored in the volume is frequently used by a user right after when the data is created, the data is not used after a period of time. Thus, there is a problem that the expensive volume is uneconomically consumed if the non-used data is continuously stored in the high-performance volume.
There has been known a data management method called a Hierarchical Storage Management (HSM) function as what solves these problems. According to the HSM function, a storage device has a plurality of volumes having different performances and a file-sharing server creates file systems in those volumes. Then, the file-sharing server moves data from the high-performance volume to the low-performance volume according to a frequency of use.
Thus, the HSM function holds a file frequently used in the “high-speed and expensive” volume and a file not used so often in the “low-speed and inexpensive” volume. Thus, the HSM function aims at cutting costs required for storage by controlling the files as described above.
According to the HSM function as disclosed in US2004/0193760A1, a file system instructs to move a file not used to the low-speed volume. Then, when an application accesses to the file after that, the file system moves the file to the low-speed volume. US2004/0193760A1 discloses that the file system rewrites file management information of the high-speed volume at this time.
According to the prior art HSM function, the file system of the high-speed volume recognizes information of destination of the file by making reference to a stub file (or management information) held in the high-speed volume. The move of the file between the high-speed volume and the low-speed volume using the HSM function is triggered when a user accesses to the file. That is, when the user accesses to the file, there arises a problem that access performance drops in moving the file because the file system copies the file to be moved from the low-speed volume to the high-speed volume. Still more, even when a super high-speed volume that is faster than the high-speed volume is used, there is a problem that performance of the super high-speed volume cannot be fully utilized because the stub file is stored in the high-speed volume.
Accordingly, the invention seeks to solve the aforementioned problems and to provide a file management method and a hierarchy management file system capable of preventing the access performance from dropping when the user accesses to a file.